Chapter Chapter 1212SolutionsSolutions

Colligative PropertiesColligative Properties colligative properties are

properties whose value depends only on the number of solute particles, and not on what they are◦ Vapor Pressure Depression,

Freezing Point Depression, Boiling Point Elevation, Osmotic Pressure

Solutions of ionic substances often have a vapor pressure significantly lower than predicted, because the ion-dipole forces between the dissolved ions and polar water molecules are so strong.

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the van’t Hoff factor, i, is the ratio of moles of solute particles to moles of formula units dissolvedmeasured van’t Hoff factors are often lower than you might expect due to ion pairing in solution

Ionic Solutes and Vapor Ionic Solutes and Vapor PressurePressure according to Raoult’s Law, the effect of solute on

the vapor pressure simply depends on the number of solute particles

when ionic compounds dissolve in water, they dissociate – so the number of solute particles is a multiple of the number of moles of formula units

the effect of ionic compounds on the vapor pressure of water is magnified by the dissociation◦ since NaCl dissociates into 2 ions, Na+ and Cl,

one mole of NaCl lowers the vapor pressure of water twice as much as 1 mole of C12H22O11 molecules would

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Example – Example – What is the vapor pressure of H2O

when 0.102 mol MgCl2 is mixed with 0.927 mol H2O @ 55°C?

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Freezing Point DepressionFreezing Point Depression the freezing point of a solution is lower than the freezing

point of the pure solvent◦ for a nonvolatile solute◦ therefore the melting point of the solid solution is lower

the difference between the freezing point of the solution and freezing point of the pure solvent is directly proportional to the molal concentration of solute particles

FPsolvent – FPsolution) = Tf = m∙Kf the proportionality constant is called the Freezing Point

Depression Constant, Kf

◦ the value of Kf depends on the solvent◦ the units of Kf are °C/m

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KKff

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Example– What is the freezing point of a Example– What is the freezing point of a 1.7 m aqueous ethylene glycol solution, 1.7 m aqueous ethylene glycol solution, CC22HH66OO22??

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Boiling Point ElevationBoiling Point Elevation the boiling point of a solution is higher than the boiling

point of the pure solvent◦ for a nonvolatile solute

the difference between the boiling point of the solution and boiling point of the pure solvent is directly proportional to the molal concentration of solute particles

BPsolution – BPsolvent) = Tb = m∙Kb the proportionality constant is called the Boiling Point

Elevation Constant, Kb

◦ the value of Kb depends on the solvent◦ the units of Kb are °C/m

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Ex 12.9 – How many g of ethylene glycol, Ex 12.9 – How many g of ethylene glycol, CC22HH66OO22, must be added to 1.0 kg H, must be added to 1.0 kg H22O to give a O to give a solution that boils at 105°C?solution that boils at 105°C?

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PROPERTIES OF SOLUTIONSPROPERTIES OF SOLUTIONSSolutions • contain small

particles (ions or molecules).

• are transparent.• do not separate.• cannot be filtered.

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Suspensions• have very large particles.• settle out. • can be filtered. • must be stirred to stay suspended.Examples include: blood platelets,

muddy water, and calamine lotion.

ColloidsColloids a colloidal suspension is a heterogeneous mixture

in which one substance is dispersed through another◦ most colloids are made of finely divided particles

suspended in a medium the difference between colloids and regular

suspensions is generally particle size – colloidal particles are from 1 to 100 nm in size

the particles in a colloid exhibit Brownian motion colloids exhibit the Tyndall Effect

◦ scattering of light as it passes through a suspension

◦ colloids scatter short wavelength (blue) light more effectively than long wavelength (red) light

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Solutions, Colloids, and Solutions, Colloids, and SuspensionsSuspensions

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OsmosisOsmosisosmosis is the flow of solvent through a semi-

permeable membrane from solution of low concentration to solution of high concentration

the amount of pressure needed to keep osmotic flow from taking place is called the osmotic pressure

the osmotic pressure, , is directly proportional to the molarity of the solute particles◦ R = 0.08206 (atm∙L)/(mol∙K)

= MRT

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Suppose a semipermeable membrane separates a 4% starch solution from a 10% starch solution. Starch is a colloid and cannot pass through the membrane, but water can. What happens?

4% starch 10% starchH2O

Ex 12.10 – What is the molar mass of a protein if Ex 12.10 – What is the molar mass of a protein if 5.87 mg per 10 mL gives an osmotic pressure of 5.87 mg per 10 mL gives an osmotic pressure of 2.45 torr at 25°C?2.45 torr at 25°C?

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ISOTONIC SOLUTIONSISOTONIC SOLUTIONSAn isotonic solution• exerts the same osmotic

pressure as red blood cells. • is known as a “physiological

solution.”• of 5.0% glucose or 0.90%

NaCl is used medically because each has a solute concentration equal to the osmotic pressure equal to red blood cells.

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H2O

HYPOTONIC SOLUTIONSHYPOTONIC SOLUTIONSA hypotonic solution • has a lower osmotic

pressure than red blood cells.

• has a lower concentration than physiological solutions.

• causes water to flow into red blood cells.

• causes hemolysis: RBCs swell and may burst.

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H2O

HYPERTONIC SOLUTIONSHYPERTONIC SOLUTIONSA hypertonic solution

• has a higher osmotic pressure than RBCs.

• has a higher concentration than physiological solutions.

• causes water to flow out of RBCs.

• cause crenation: RBCs shrink in size.

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H2O

ColloidsColloidsa colloidal suspension is a heterogeneous

mixture in which one substance is dispersed through another◦ most colloids are made of finely divided

particles suspended in a medium the difference between colloids and regular

suspensions is generally particle size – colloidal particles are from 1 to 100 nm in size

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